Carrier aggregation for Long Term Evolution (LTE) was introduced in 3rd Generation Partnership Project (3GPP) Release 10. This feature allows a wireless transmit/receive unit (WTRU) to transmit and receive on more than one carrier simultaneously, resulting in an increase of its peak data rate over the air interface. The maximum number of carriers that may be aggregated is five (5), for a maximum bandwidth of 100 megahertz (MHz).
Transmission of data in the downlink in LTE may be performed using the physical downlink shared channel (PDSCH). This physical channel supports hybrid automatic repeat request (HARQ) transmission, in which the receiver (at the WTRU) may combine successive transmissions of a transport block to increase the probability of successful decoding at each retransmission. The WTRU may report, for a given reception attempt of a transport block, that the reception has succeeded with an acknowledgement (ACK) or has not succeeded with a negative acknowledgement (NACK). In some cases the WTRU may also report that it did not detect that a transport block was transmitted, such as in discontinuous transmission (DTX).